In a steam generator the oxidization heat of a fuel is transferred to a heat carrying medium. The transfer takes place by way of evaporator heating surfaces, in particular by way of tubes, through which the heat carrying medium is conducted and which—generally grouped in blocks—are passed through a combustion chamber or around it. Smooth tubes or tubes with inner ribbing are generally used. The ribs of tubes with inner ribbing, which are provided to create or assist swirling in the flow medium and generally have the form of a helix or screw thread, may be necessary for a number of reasons. These include in particular a low full load mass flow density of the evaporator, a high heat flow density and the resulting risk of film boiling or avoiding flow stratification, which can occur more readily on smooth surfaces than on rough or structured surfaces.
Tubes with inner ribbing are generally produced from a single piece by means of a cold drawing process. However such a cold drawing process has limitations in respect of the composition of the raw material used. For steel alloys for example a chromium content of 5% represents an upper limit for tube production by means of a cold drawing process, as higher chromium contents restrict the shaping capacity of the steel alloy too much.
On the other hand certain operating parameters of an evaporation process may require or advantageously utilize specific material properties, which cannot be achieved with the material compositions that can be permitted for the cold drawing process. For example alloyed steels with chromium contents beyond 5% are required where there are particularly high heat flow densities and/or particularly high temperatures of the heat carrying medium due to the resulting high tube wall temperatures.
Therefore cold drawn tubes with inner ribbing can be replaced by smooth tubes provided with corresponding inserted elements for such applications. One class of such inserted elements is provided for example by helically wound wires or wire mesh, which is/are produced with the aid of a former shaft and inserted into a smooth tube and which rest against the inner surface of the smooth tube so that contact is made due to their torsional stress. To prevent displacement and creep, it is necessary to attach such inserted elements at selected points on the inside of the smooth tube. This is generally achieved by spot welding. As the tube represents a hollow member and the weld points are located on the inside of the tube and are therefore difficult to see, a suitable control method is required for spot welding.